This invention relates to an anti-backdraft shutter assembly to prevent the backflow of air through an axial flow fan as may, for example, be fitted to an evaporative cooler fixed to the roof of a building.
Throughout this description and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d will be understood to imply the inclusion of a stated integer or step or group of integers or steps.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.
An evaporative cooler of a type designed for the cooling of an entire building, is generally mounted in a fixed location on the roof of the building. When operating, the evaporative cooler draws air from outside the building, cools the air by drawing it over wetted pads, and distributes cooled air throughout the building via a system of ductwork, or through a distribution plenum. However, when the evaporative cooler is turned off, there exists an almost free air passage back through the ductwork and through the evaporative cooler which will allow air within the building to flow to the outside. Any pressure difference between the inside of the building and the outside will allow a flow of air through this free air passage of the evaporative cooler, thus creating a draft within the building.
This action is particularly troublesome in winter when the evaporative cooler is generally not being used. When the air in the building is heated, warm air rises to the roof or ceiling due to convection. The warm air will flow out of the building through the evaporative cooler unless some means is installed for interrupting its flow.
Traditionally, evaporative coolers have been fitted with external covers to make them as close as practicable to airtight during the winter. While this method does stop the flow of warm air out of the building, it does have a number of disadvantages. The cover requires the owner or agent to climb onto the roof to install. The cover is generally not installed until the weather is consistently cold, leaving many days when warm air can escape from the building but the weather does not warrant fitting the cover. Once installed, the cover is difficult and inconvenient to remove should there be more warm weather. The cover is subjected to the elements of the weather and generally has a short service life.
These disadvantages can be overcome by fitting an automatically operated flap beneath the fan which flap operates such that it is automatically opened when the fan is turned on, and automatically closes when the fan is turned off. When closed, the flap provides an almost airtight seal. Such devices have been offered as features in evaporative coolers in recent years, and one such device is described in our existing Australian Patent No. 693860 and its U.S. counterpart U.S. Pat. No. 5,567,114. The mechanism in our existing Patent consists of a counterbalanced flap which covers the delivery opening of a centrifugal fan when in the closed position. When the fan is operating, the counterbalanced flap is pushed into the open position and held there by the air pressure applied by the fan. That device overcomes all of the deficiencies described above relating to an external cover, and does not disturb the flow of air from the fan. However, while that device can readily be installed on a centrifugal fan, it is not readily adaptable for use on an axial flow fan.
An axial flow fan is somewhat more difficult to seal automatically without installing the sealing mechanism and its support in the airflow, hence disturbing the flow of air from the fan. Many devices which provide automatic sealing have been described. Examples include U.S. Pat. No. 4,385,552 xe2x80x9cExhaust fan and anti-backdraft shutter assemblyxe2x80x9d, U.S. Pat. No. 3,965,926 xe2x80x9cFlapper valve with independent plate suspensionxe2x80x9d, U.S. Pat. No. 4,257,444 xe2x80x9cInsert check valve or the likexe2x80x9d, U.S. 4,257,451 xe2x80x9cCheck valvexe2x80x9d and U.S. Pat. No. 3,960,464 xe2x80x9cImprovements in and relating to fansxe2x80x9d. The devices described invariably support their operating mechanism in the airflow of the fan, and require a spring mechanism to return the flaps to the closed position. All of these mechanisms result in degradation of the airflow from the fan due to the disturbance of the flow provided by the support structure of the mechanism in the airstream immediately following the fan. The use of springs as the means of closure of the flaps results in the maximum resistance to opening of the flaps in the full open position, when the flap will have the maximum disturbance to the airflow unless completely removed from the air stream. Since complete removal from the airstream would require a separate mechanism, the use of springs for flap return invariably results in incomplete removal of the flaps from the airstream in the open position, and further degradation of the air delivery of the fan. This effect is exacerbated if the fan is run at less than its maximum speed.
The present invention overcomes all of the disadvantages of the devices in the prior art. A flap support mechanism is out of the airstream. At least one flap is provided to close off a duct downstream of the fan. The at least one flap being counterbalanced by a lever and a counterweight to remain in the open position when a minimum of airflow is provided by the fan. When the fan is turned off, the unbalanced counterweight forces cause the flap to proceed towards the closed position, with the counterweight out of balance force increasing towards the closed position due to a change in lever arm of the counterweight. In the fully closed position, the holding force of the counterweight is preferably supplemented by a magnetic catch to ensure the flap is not blown open by wind external to the evaporative cooler. Once opened by operation of the fan, the flap remains in the full open position with virtually no disturbance to the air flow, even when the fan is run at part speed.
An anti-draft shutter assembly for an axial flow fan fitted to a duct, said assembly comprising at least one counterbalanced flap supported downstream of airflow from the fan when in operation, said at least one flap being pivotally supported outside the perimeter of the duct, when in situ, and being biased by a counterbalance weight toward a duct closing position to close the duct when the fan is not operating. Preferably, the at least one flap is held in the closed position by a magnetic catch on the at least one flap.